A Class-D switching amplifier, which is often desirable for amplification of audio signals, is substantially similar to Class-A, Class-B, and Class-AB, with a major difference in the signals provided to the output stage. Instead of feeding the audio waveform directly to the output stage, Class-D amplifiers modulate the audio waveforms as on-off pulses using duty-cycle modulation methods such as Pulse Duty-Cycle Modulation (PDM) or Pulse Width Modulation (PWM), before feeding the signal to the output stage.
By using transistors and semiconductors as switches rather than as linear amplifiers, the modulation stage rapidly switches the output stage on and off with the width, in the case of PWM, varying as a function of the audio signal. Subsequently, sound is recreated by filtering the signal—usually by low-pass filtering the switching signal—at the output, resulting in an amplified version of the analog input signal. Class-D amplifiers typically use triangular reference waveform as the comparison signal for modulation. In practice, high-frequency modulation is required to make a smooth waveform at the speaker. The switching scheme makes Class-D amplifiers more efficient and smaller in size, with less wasted heat energy and a smaller power supply. Class-D amplifiers are much more efficient than the nonswitching linear amplifiers.
Existing Class-D amplifiers suffer numerous shortcomings in areas including modulation, feedback, distortion, power supply ripple rejection, response time, isolation, and last stage filtering. The triangular waveform, for modulation purposes, by itself is the cause of several problems in Class-D amplifiers such as superimposed high frequency noise, which is a source of distortion. Pulse transient damping issues, at frequencies above 1 kHz are another source of distortion in Class-D amplifiers.
As a result of the above-mentioned problems and other identified disadvantages in the art, there is a need for an improved Class-D audio amplifier without the triangular or other reference input signal that can operate in the several hundred kilohertz range.